The employment of magnetoresistive (MR) sensors for reading signals from media is well known. Such sensors read signals from the media by detecting a change in resistance of the sensor due to magnetic fields from the media. By fundamental principles of magnetoresistance, three general kinds of magnetoresistive (MR) sensors have been developed: an AMR sensor that utilizes the anisotropic magnetoresistive effect, such as a soft adjacent layer (SAL) sensor; a GMR sensor or GMR spin valve that utilizes the giant magnetoresistive effect; and a TMR sensor that utilizes the tunneling magnetoresistive effect.
MR sensors can be categorized based upon the direction of flow of the current that is used to detect the change in resistance of the sensor. It is known for this sense current to be directed either generally along the plane of the sensor layers or generally perpendicular to the plane of the sensor layers. The former configuration may be known as a current-in-plane (CIP) sensor, and the latter configuration may be known as a current-perpendicular-to-plane (CPP) sensor. In CIP sensors, conductive leads for the sensor are disposed between a pair of magnetically soft shield layers, and separated from the shields by electrically insulating read gap layers that are typically greater than five hundred angstroms in thickness. CPP sensors are instead electrically connected between the shields, which may serve as leads for the sensor.
CIP spin valve sensors typically include two ferromagnetic layers that are separated from each other by a nonferromagnetic, electrically conductive spacer layer. One of the ferromagnetic layers has a magnetization that is fixed or pinned in one direction (so-called “pinned layer”), and the other ferromagnetic layer has a magnetization that is free to rotate in the presence of a magnetic field (so-called “free layer”). CPP spin-dependent tunneling sensors may also have a free layer separated from a pinned layer, but the spacer layer in this case may be formed of electrically insulating material such as alumina or aluminum-nitride, formed to a thickness that is small enough (e.g., less than ten angstroms) to allow measurable quantum mechanical tunneling.
For both of the above sensors, the magnetization direction of the free layer relative to the pinned layer affects the resistance of the sensor, so that the resistance of the sensor senses the magnetic field from the media. The resistance of the sensor can itself be measured via changes in either voltage or electrical current while the sensor is subjected to a bias current or a bias voltage, respectively. The magnetization of the pinned layer can be fixed by an adjoining antiferromagnetic layer via exchange coupling. Tantalum is a conventionally preferred seed layer material for a subsequently deposited antiferromagnetic layer, in part due to the ability of tantalum films to induce a strong (111) crystalline structure in the subsequently deposited sensor layers. Such a (111) crystalline structure in the sensor layers was conventionally believed to be important in obtaining high values for parameters of importance such as the magnetoresistance. Stated differently, a decrease in the (111) texture was generally correlated with a decrease in magnetoresistance. Despite the fabrication of sensors with strong (111) textures, a need still exists for improved magnetoresistance.